1. Field of the Invention
The present invention relates to a swivel chair with adjustable tilt. It relates more particularly to a swivel chair having a cantilever, a seat that is pivotally and slidably connected to the cantilever, a backrest carrier pivotally connected to the seat and the cantilever, and springs connected between the cantilever and the seat so as to controllably contain the sliding of the cantilever with respect to the seat, and thus controllably contain the tilting of the backrest and seat of the chair.
2. Description of the Prior Art
German Offenlegungsschrift No. 29 25 520 describes a swivel chair in which a compact arrangement of the spring elements results in a space-saving design and a substantial simplification of the actuation lever system for the springs. A pivot bearing on the backrest carrier frame links it to the rear end of the seat support. The seat support is, in turn, pivotally mounted at its front end to the front end of a seat cantilever, which is connected at its rear end to the chair column. Also, horizontal struts at the lower end of the backrest carrier frame are linked by slide guides to the cantilever. This structure provides an advantageous tilt relationship between the seat, the back rest, and the other tilt members for proper back and pelvic support in all tilt positions. When the chair is tilted the seat support pivots about its mounting pivoting point on the cantilever, and the rear of the seat tilts downward as the backrest support tilts back.
Notwithstanding the advantages of such a swivel chair, its construction is relatively complicated and thus expensive. The backrest carrier is connected by slide guides to the cantilever and by a pivot mount to the seat support, causing the seat to tilt relatively little and pelvic support to remain substantially unchanged when the back is tilted. Keeping the position of the legs and pelvis unchanged while the back and seat are tilted can result in discomfort. When the seat and the chair back are pivoted, there should be a corresponding change in the location and direction of pelvic support.
The same holds true for the chair as described in U.S. Pat. No. 4,384,741, which corresponds to German Offenlegungsschrift No. 27 48 680. This patent employs mechanical tension springs rather than gas pressure springs as in the German Offenlegungsschrift No. 29 25 520 patent. The springs pivotally engage the front seat shell and the backrest carrier frame. Because of the particular lever conditions in such a spring arrangement, relatively strong springs must be used, which can impair tilting ease and seating comfort. Moreover, very short spring extensions are provided between the backrest carrier and the seat shell, which likewise necessitates the use of strong springs. With seat shell suspensions that are pivotable only in the front area, a double articulation must be employed between the rear of the seat shell and the backrest carrier. When the chair is tilted back into its farthest tilt position, this causes a substantial displacement between the rear edge of the seat shell and the lower backrest edge, which greatly impairs seating comfort and stretches the chair's upholstery.